The present invention is directed, in general, to driver circuits for light emitting diode illumination sources and, more specifically, to voltage-controlled dimming driver circuits for light emitting diode illumination sources employed in place of incandescent lamps within aircraft crewstation instrumentation.
Commercial and military aircraft instrumentation displays, like many other display systems, frequently employ illuminated indicators and controls. Traditionally, incandescent lamps operating at 5 VAC, 14 VDC or 28 VDC have been employed as illumination sources for illuminated pushbutton switches, indicators and annunciators within aircraft instrumentation. The illumination from such incandescent lamps is generally optically filtered to produce a wide range of human visible or night vision imaging system (NVIS) colors, and the small size of incandescent lamps allows multiple lamps to be used within the same display to illuminate different regions of the display in different colors.
The luminance required of incandescent displays varies from approximately 400 foot-lamberts at full rated voltage for sunlight-readability in daytime flying to 15 foot-lamberts for commercial/general aviation night flying, 1.0 foot-lamberts for military night flying, and 1.0 foot-lamberts for night flying utilizing NVIS night vision goggles. Because the luminance of incandescent lamps varies with applied voltage within a certain range, output luminance levels of displays are adjusted for night flying conditions by reducing the supplied voltage to approximately one-half or less of the normal full rated operating voltage (i.e. voltage-controlled dimming).
The inherent characteristics of incandescent lamps, however, lead to noticeable chromaticity shifts as the applied voltage is reduced. Moreover, incandescent lamps suffer other disadvantages when employed in aircraft instrumentation, including high power consumption, high inrush current, uncomfortably high touch temperatures, and unreliability in high vibration environments. As a result, considerable effort has been expended to incorporate more stable, efficient and reliable technologies, such as light emitting diodes (LEDs), into aircraft crewstation illuminated displays, and to retrofit existing displays.
The use of light emitting diodes as a retrofit in illuminated displays for aircraft crewstation instrumentation generally requires connection to aircraft wiring, circuitry and systems originally designed to operate with incandescent lamps. However, light emitting diodesxe2x80x94unlike incandescent lampsxe2x80x94can produce low but detectable levels of illumination with as little as a few microamperes (xcexcA) of current. For a variety of reasons, currents at such levels exist in aircraft wiring and avionics boxes coupled to illuminated displays when the displays are not supposed to be illuminated, and may result in inadvertent or unintentional illumination when light emitting diodes are employed as an illumination source. Experimentation has revealed that indium gallium nitride light emitting diodes (blue, green, or yellow, depending on the indium concentration, or white if packaged with phosphor) are particularly vulnerable to such inadvertent low luminance levels.
Because incandescent lamps were essentially immune to inadvertent illumination while light emitting diodes are not, additional driver circuitry is required for light emitting diodes to prevent inadvertent illumination. Requiring a minimum current of 1.0 milliamperes (mA) to illuminate the light emitting diode(s) has been determined through experimentation to be sufficient to prevent inadvertent illumination, even when a few hundred microamperes (xcexcA) of current are unintentionally generated across the light emitting diode driver inputs.
For example, a typical light emitting diode driver circuit for employing light emitting diodes as illumination sources in retrofitting aircraft instrumentation is shown in FIG. 3. Driver 300 includes a biasing resistor R2 and a light emitting diode L1 connected in series between input and output ports (xe2x80x9c+xe2x80x9d and xe2x80x9cxe2x88x92xe2x80x9d) to which the input voltage is applied. For an input voltage of 28 VDC, a typical resistance value for resistor R2 would be 1250 ohms (xcexa9), resulting in a forward voltage drop of approximately 3.0 VDC across light emitting diode L1 and a current through resistor R2 and light emitting diode L1 of approximately 20 mA. For night flying conditions, the applied input voltage across the input and output ports is reduced to a level where the forward voltage drop across light emitting diode L1 is approximately 2.37 VDC and the total circuit current is approximately 50 xcexcA. This 50 xcexcA circuit current is a level known to be vulnerable to inadvertent illumination, rendering the driver 300 unsuitable.
There is, therefore, a need in the art for quiescent current limiting in light emitting diode driver circuits employed for aircraft crewstation instrumentation, and particularly power efficient quiescent current limiting.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in voltage-controlled dimming light emitting diode driver, a quiescent current limiting mechanism to prevent inadvertent illumination of a light emitting diode (or set of light emitting diodes) by stray currents at extremely low levels, which is implemented in the present invention by a resistive load connected in parallel with the light emitting diode. The quiescent current limiting resistive load is sized to conduct a desired minimum current at the lowest forward voltage drop at which the light emitting diode is expected to properly illuminate. Rather than connecting the resistive load across the input/output ports of the driver circuit, in parallel with any biasing resistance and the light emitting diode, the load is connected directly in parallel with the light emitting diode. Additional current through the quiescent current limiting resistive load as the voltage across the input/output ports increase is thus effectively capped by the maximum forward voltage drop across the light emitting diodes.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9corxe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.